Current-to-voltage converting apparatus and impedance measuring apparatus

ABSTRACT

A current-to-voltage converting apparatus connected to an element or a circuit having a first terminal connected to a signal source and comprising a feedback amplifier, which is connected to a second terminal of the element or the circuit and keeps the second terminal at virtual ground, and which converts the current signals that flow to the element or the circuit to voltage signals and outputs these signals; a device for opening the feedback loop of the feedback amplifier and measuring the open-loop loss of the feedback loop; and a compensating amplifier, which compensates for the open-loop loss. It further comprises a device for measuring the open-loop phase shift of the feedback loop when the feedback loop is open and a control unit for keeping the open-loop phase shift at a pre-determined value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to an impedance measuring apparatus andin particular, relates to an impedance measuring apparatus with whichhigh-speed measurement is possible.

2. Background of the Art

Impedance measuring apparatuses that operate by the automaticbalanced-bridge method are an example of the prior art of impedancemeasuring apparatuses. Impedance measuring apparatuses that operate bythe automatic balanced-bridge method are characterized in that theycover a broad measurement frequency range and their measurement accuracyis good within a broad impedance measurement range.

The internal structure and operation of an impedance apparatus thatoperates by the automatic balanced-bridge method are described below.FIG. 1 is a drawing showing the internal structure of an impedancemeasuring apparatus that operates by the automatic balanced-bridgemethod. Impedance measuring apparatus 10 in FIG. 1 comprises signalsource 200, current-to-voltage converting apparatus 300, and vectorvoltmeter 400 for determining the impedance of device under test 100.The entire impedance measuring apparatus 10 is operated under thecontrol of an operation control device CTRL₁ (not illustrated), such asa CPU.

Device under test 100 is an element or a circuit having two terminals.Device under test 100 should have at least two terminals and also, itcan be an element or a circuit with three or more terminals. In thiscase, two of the three or more terminals are used for the measurements.Device under test 100 is represented by “DUT” in FIG. 1. The point wheredevice under test 100, cable 510, and cable 520 are connected in FIG. 1is referred to as the High terminal. Moreover, the point where deviceunder test 100, cable 530, and cable 540 are connected is referred to asthe Low terminal.

Signal source 200 is the signal source that is connected to a firstterminal of the device under test 100 by cable 510 and generatesmeasurement signals that are applied to device under test 100. Moreover,signal source 200 is also connected to vector voltmeter 400 by cable510, cable 520, and buffer 550 and feeds measurement signals to vectorvoltmeter 400. The measurement signals are single sine-wave signals.However, the measurement signals are not limited to single sine-wavesignals and can also be signals that comprise several sine waves.

Current-to-voltage converting apparatus 300 converts the current thatflows to device under test 100 and outputs voltage signals to buffer560. Current-to-voltage converting apparatus 300 comprises a nulldetector 310, a narrow-band amplifier 600, a buffer 320, and a rangeresistor 330. Cable 530, null detector 310, narrow-band amplifier 600,buffer 320, range resistor 330, and cable 540 form a negative feedbackloop 340.

Null detector 310 balances the current that flows to range resistor 330and the current that flows to device under test 100 and outputs signalsto narrow-band amplifier 600 such that the current that flows into theinput terminals of null detector 310 through cable 530 will be broughtto zero. When the current that flows to range resistor 330 and thecurrent that flows to device under test 100 are balanced, the current atthe Low terminal is kept at virtual ground.

FIG. 2 is a drawing showing the internal structure of narrow-bandamplifier 600. Narrow-band amplifier 600 comprises a phase sensitivedetector 610, a filter 620 and a filter 630, as well as a vectormodulator 640, and amplifiers and amplifies the output signals of nulldetector 310 and outputs them to buffer 320. Narrow-band amplifier 600resolves the output signals of null detector 310 into an in-phasecomponent and an quadrature-phase component using phase sensitivedetector 610, filters the in-phase component and quadrature-phasecomponent using filter 620 and filter 630, modulates the filteredin-phase component and quadrature-phase component using vector modulator640, and feeds the vector-modulated voltage signals to buffer 320.

Phase sensitive detector 610 is a quadrature detector and comprises amixer 611, a mixer 612, a signal source 613, and a signal source 614.Signal source 613 generates sine-wave signals and feeds them to mixer611. Moreover, signal source 614 generates cosine-wave signals and feedsthem to mixer 612. The sine-wave signals output by signal source 613 andthe cosine signals output by signal source 614 have the same frequencyas the measurement signals and they are orthogonal to each other.Consequently, mixer 611 and mixer 612 can orthogonally resolve theoutput signal of null detector 310 into an in-phase component and anquadrature-phase component.

Filter 620 is an integrator that comprises a resistor 621, an amplifier622, and a capacitor 623, and integrates the output signals of mixer611. Filter 630 is an integrator comprising a resistor 631, an amplifier632, and a capacitor 633, and integrates the output signals of mixer612.

Vector modulator 640 comprises a mixer 641, a mixer 642, a signal source643, a signal source 644, and an adder 645. Signal source 643 generatessine-wave signals and feeds them to mixer 641. Moreover, signal source644 generates cosine signals and feeds them to mixer 642. The sine-wavesignals output by signal source 643 and the cosine-wave signals outputby signal source 644 have the same frequency as the measurement signals,and they are orthogonal to each other. Mixer 641 modulates the sine-wavesignals that are output from signal source 643 with the output signalsof filter 620 and outputs the modulated sine signal. Mixer 642 modulatesthe cosine-wave signals output from signal source 644 with the outputsignals of filter 630 and outputs the modulated cosine signal. Thevoltage signals output from mixer 641 and the voltage signals outputfrom mixer 642 are added by adder 645 and output to buffer 320.

Vector voltmeter 400 of FIG. 1 measures output signal E_(dut) of buffer550 and output signal E_(rr) of buffer 560. Control device CTRL₁calculates the vector ratio of signal E_(dut) and signal E_(rr) thathave been measured and calculates the impedance of device under test 100from the calculated vector ratio and the resistance of range resistor330.

Measurement of the gate oxide film is one important measurement in theproduction of MOS devices. The gate oxide film thickness is an importantparameter in determining the operating threshold of MOS-type devices.The gate oxide film thickness is measured by measuring the impedance ofan MOS device, calculating the capacitance from the impedancemeasurement, and converting this calculated capacitance to theequivalent oxide film thickness using the dielectric constant.

When an MOS device on a semiconductor wafer is tested using aconventional impedance measuring apparatus 10, a wafer interface devicecomprising a switch matrix, a chuck, a probe card, and the like is addedbetween the impedance measuring apparatus 10 and device under test 100.The wafer interface device has a larger ground capacitance than deviceunder test 100. Moreover, cable 510, cable 520, cable 530, and cable 540that are connected between this wafer interface device and impedancemeasuring apparatus 10 are relatively long and also have a large groundcapacitance. Cable 510, cable 520, cable 530, and cable 540 are calledcable 510, etc., hereafter. FIG. 3 is a drawing in which theabove-mentioned ground capacitance has been added to FIG. 1. C_(cable)in FIG. 3 is the total ground capacitance of cable 510, etc. Moreover,C_(winf) is the ground capacitance of the wafer interface device. Theground capacitance of the wafer interface device comprises the groundcapacitance of the switch matrix, the ground capacitance of the chuck,and the ground capacitance of the probe card.

Conventional impedance measuring apparatus has two problems withhigh-speed measurements. The first problem is that when a large groundcapacitance is applied to the Low terminal, the current-to-voltageconverting apparatus 300 takes a long time to settle. If the time tosettling of the current-to-voltage converting apparatus 300 is long, thetime until the current that flows to range resistor 330 and the currentthat flows to the device under test are balanced is also long and thewait time until measurements begin is increased. When the capacitance ofan MOS device on a semiconductor wafer is measured, this problem isexacerbated by a wafer interface device and cable 510, etc., with alarge ground capacitance, as described above.

The second problem is that when the capacitance of an MOS device on asemiconductor wafer is measured, the ground capacitance of the waferinterface device and cable 510, etc. is not constant. There are manytypes of wafer interface devices and cable 510, etc. depending on thedevice under test and the user's selection. Consequently, the groundcapacitance of the wafer interface device and cable 510, etc. is notconstant. Unless the ground capacitance on the wafer interface deviceand cable 510, etc. is constant, it will be very difficult to keep theground capacitance from affecting the measurement results as planned.

There has been considerable progress in microfabrication technology forsemiconductors in recent years, with a huge number of elements orcircuits being formed on one wafer. While there has been an obviousincrease in the number of elements that serve as the device under test,a corresponding increase in measurement time is not allowed. Moreover,sacrifice of measurement precision for high-speed measurement is notacceptable. The realization of high-speed, high-precision impedancemeasurement is a very important problem in the semiconductor industrytoday.

SUMMARY OF THE INVENTION

The present invention provides a novel apparatus with which impedancecan be measured at high speed and high precision in order to solve theabove-mentioned problems.

The present invention was created in order to realize theabove-mentioned object. The present invention is characterized in thatit is a current-to-voltage converting apparatus connected to an elementor a circuit having a first terminal connected to a signal sourcecomprising a feedback amplifier, which is connected to a second terminalof this element or this circuit and keeps this second terminal atvirtual ground, and converts current signals that flow to this elementor this circuit to voltage signals and outputs these signals; means foropening the feedback loop of this feedback amplifier and measuring theopen-loop loss of this feedback loop; and a compensating amplifier,which compensates for this open-loop loss.

Moreover, the present invention also provides an impedance measuringapparatus characterized in that it comprises a signal source connectedto a first terminal of a device under test; a feedback amplifier, whichis connected to a second terminal of this device under test and keepsthis second terminal at virtual ground, and converts to voltage signalsand outputs the current signals that flow to this device under test;means for opening the feedback loop of this feedback amplifier andmeasuring the open-loop loss of this feedback loop; a compensatingamplifier, which compensates this open-loop loss; and means formeasuring the vector voltage ratio between the voltage signals betweenthis first terminal and this second terminal and the output signals ofthis feedback amplifier; and it measures the impedance of this deviceunder test from this vector voltage ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing the internal structure of an impedancemeasuring apparatus of the prior art.

FIG. 2 is a drawing showing the internal structure of a narrow-bandamplifier of an impedance measuring apparatus of the prior art.

FIG. 3 is a drawing showing an impedance measuring apparatus of theprior art to which a wafer interface apparatus has been added.

FIG. 4 is a drawing showing the internal structure of the impedancemeasuring apparatus of the present invention.

FIG. 5 is a drawing showing the internal structure of the narrow-bandamplifier of the impedance measuring apparatus of the present invention.

FIG. 6 is a flow chart showing the operation of the impedance measuringapparatus of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described based on the preferredembodiments shown in the appended drawings. The first embodiment of thepresent invention is an impedance measuring apparatus that operates bythe automatic balanced-bridge method, and its internal structure isshown in FIG. 4. The same reference symbols are used in FIGS. 1 and 4for structural elements having the equivalent function and properties.

An impedance measuring apparatus 20 in FIG. 4 comprises a signal source200, a current-to-voltage converting apparatus 800, and a vectorvoltmeter 400 for measuring the impedance of device under test 100.Impedance measuring apparatus 20 operates under the control of computerdevice CTRL₂ (not illustrated) that executes the programs.

Device under test 100 comprises multiple MOS devices on a semiconductorwafer. For convenience, the device under test is represented in thedrawing as only one “DUT.” The MOS capacitance of the MOS device ismeasured in the present embodiment and therefore, device under test 100is a capacitor with a first terminal and a second terminal. Device undertest 100 is connected to impedance measuring apparatus 20 through awafer interface device 700. Although not illustrated, wafer interfacedevice 700 comprises a switch matrix, a chuck, a probe card, and thelike. The point where wafer interface device 700 and a cable 510 and acable 520 are connected is referred to as the High terminal. Moreover,the point where wafer interface device 700 and cable 530 and a cable 540are connected is referred to as the Low terminal. Device under test 100should have at least two terminals and also can be an element or circuitwith three or more terminals, such as a transistor. In this case, two ofthe three or more terminals are used in the measurements.

Signal source 200 is the signal source that is connected to the firstterminal of device under test 100 via cable 510 and wafer interfacedevice 700 and generates the measurement signals that will be applied todevice under test 100. Moreover, signal source 200 is the signal sourcethat is connected to vector voltmeter 400 through cable 510, cable 520,and buffer 550 and feeds the measurement signals to vector voltmeter400. The measurement signals are single sine-wave signals. However, themeasurement signals are not limited to single sine-wave signals and canalso be signals that comprise multiple sine waves.

A current-to-voltage converting apparatus 800 converts current flowingto device under test 100 and outputs voltage signals to buffer 560.Current-to-voltage converting apparatus 800 comprises a null detector310, a narrow-band amplifier 900, a buffer 320, and a range resistor330. Cable 530, null detector 310, narrow-band amplifier 900, buffer320, range resistor 330, and cable 540 form a negative feedback loop810.

Null detector 310 balances the current that flows to range resistor 330and the current that flows to device under test 100 and outputs signalsto narrow-band amplifier 900 so that the current that flows throughcable 530 to the input terminal of null detector 310 is brought to zero.When the current that flows to range resistor 330 and the current thatflows to device under test 100 are balanced, the voltage of the Lowterminal is kept at virtual ground.

FIG. 5 is a drawing showing the internal structure of narrow-bandamplifier 900. Narrow-band amplifier 900 comprises a phase sensitivedetector 910, filters 920 and 930, a variable gain amplifier 941, avariable gain amplifier 942, a switch 951, a switch 952, a constantvoltage source 961, a constant voltage source 962, a vector modulator970, a switch 980, and a switch 990, and amplifies the output signals ofnull detector 310 and outputs them to buffer 320.

Phase sensitive detector 910 is quadrature detector and comprises amixer 911, a mixer 912, a signal source 913, and a signal source 914.Signal source 913 generates sine-wave signals and feeds them to mixer911. Moreover, signal source 914 generates cosine signals and feeds themto mixer 912. The sine-wave signals output by signal source 913 and thecosine-wave signals output by signal source 914 have the same frequencyas the measurement signals and the signals are orthogonal to each other.Consequently, mixer 911 and mixer 912 orthogonally resolve the outputsignal of null detector 310 into an in-phase component and anquadrature-phase component and output the signals to filter 920 andfilter 930. The output signal of signal source 913 and the output signalof signal source 914 should be signals that have the same frequency asthe measurement signal and are orthogonal to each other, and they can bea rectangular-wave signal rather than a sine-wave signal.

Filter 920 is an integrator comprising a resistor 921, an amplifier 922,and a capacitor 923, and integrates the output signals of mixer 911.Moreover, filter 930 is an integrator comprising a resistor 931, anamplifier 932, and a capacitor 933, and integrates the output signals ofmixer 912.

Variable gain amplifier 941 amplifies the output signals of filter 920and outputs them to switch 951. Moreover, variable gain amplifier 942amplifies the output signals of filter 930 and outputs them to switch952. The gain of variable gain amplifier 941 and that of variable gainamplifier 942 are the same and the gain is changed by computer controlCTRL₂.

Switch 951 selects either the output signals of variable gain amplifier941 or the output signals of constant voltage source 961 and outputsthese to vector modulator 970. Moreover, switch 952 selects either theoutput signals of variable gain amplifier 942 or the output signals ofconstant voltage source 962 and outputs them to vector modulator 970.

Vector modulator 970 comprises a mixer 971, a mixer 972, a signal source973, a signal source 974, and an adder 975. Signal source 973 generatessine-wave signals and feeds them to mixer 971. Moreover, signal source974 generates cosine-wave signals and feeds them to mixer 972. Thesine-wave signals output by signal source 973 and the cosine-wavesignals output by signal source 974 have the same frequency as themeasurement signals and are orthogonal to each other. Mixer 971modulates the sine-wave signals output from signal source 973 with theoutput signals of switch 951 and outputs the modulated sine signal.Mixer 972 modulates the cosine-wave signals output from signal source974 with the output signals of switch 952 and outputs the modulatedcosine signal. The voltage signals that are output from mixer 971 andthe voltage signals that are output from mixer 972 are added by adder975 and output to buffer 320. The output signals of signal source 973and the output signals of signal source 974 should be signals having thesame frequency as the measurement signals and that are orthogonal oneanother. They are not limited to sine-wave signals or cosine-wavesignal. For instance, rectangular-wave signals can be used in place ofthese signals.

Switch 980 feeds the signals that will be input to phase sensitivedetector 910 to vector voltmeter 400 as necessary. Moreover, switch 990feeds the output signals of mixer 971 to vector voltmeter 400 asnecessary.

Vector voltmeter 400 of FIG. 4 measures output signal E_(dut) of buffer550 and output signal E_(rr) of buffer 560. Control device CTRL₂calculates the vector ratio of measured signal E_(dut) and signal E_(rr)and further, calculates the impedance of device under test 100 from thecalculated vector ratio and the resistance of range resistor 330.Although not illustrated, range resistor 330 comprises multipleresistors with different resistances and selects the resistor as neededin accordance with the impedance of device under test 100 that is to bemeasured. Impedance measuring apparatus 20 thereby can measure theimpedance from a wide range of values.

Next, the operating procedure of impedance measuring apparatus 20 willbe described. As previously explained, impedance measuring apparatus 20is operated under the-control of computer device CTRL₂ that executes theprograms. Consequently, the following operating procedure describes theflow of the program executed by computer device CTRL₂. The flow chartthat shows the operating procedure of impedance measuring apparatus 20is shown in FIG. 6.

First, at step 10, impedance measuring apparatus 20 initializes theentire device. For instance, it performs voltage offset adjustmentwithin the apparatus, and the like.

Next, at step 20, negative feedback loop 810 is opened, the adjustmentsignals for measuring the open-loop loss and the open-loop phase shiftof the negative feedback loop are output and the adjustment signals(original signals) are measured. The open-loop loss and open-loop phaseshift are the loss and the phase of the one-loop transmission function.Specifically, the output signals of signal source 200 become zero ordirect current signals constant-voltage source 961 and mixer 971 areconducted with switch 951 as the A side and constant voltage source 962and mixer 972 are conducted with switch 952 as the A side. When signalsource 200 becomes either zero or a direct-current signal, the Highterminal is grounded. Sine-wave signals are output from signal source973. Zero or direct current signals are output from signal sources 974.The output signals of mixer 971 are used as the adjustment signals forthis condition. Furthermore, switch 990 is turned on and the vectorvoltage of the adjustment signals is measured by vector voltmeter 400.

Next, in step 30, the signals for adjustment that have gone through onenegative feedback loop are measured with negative feedback loop 810 leftopen. Specifically, switch 990 is turned off and switch 980 is turnedon. Moreover, the vector voltage of the signals for adjustment that havegone through one negative feedback loop is measured by vector voltmeter400.

Next, at step 40, the open-loop loss and the open-loop phase shift arecalculated, the open-loop loss is compensated, and the open-loop phaseshift is brought to the pre-determined value. Specifically, the ratio ofthe amplitude of the vector voltage and the difference in the phaseangles of the vector voltage are found by comparing the vector voltagemeasured at step 20 and the vector voltage measured at step 30. Theratio of the amplitude of the vector voltage is the open-loop loss andthe difference in the phase angles of the vector voltage is theopen-loop phase shift. In order to compensate for the open-loop loss,the gain of the variable gain amplifier 941 and the gain of variablegain amplifier 942 are set by being multiplied by the inverse of theopen-loop loss. Moreover, the phase of the output signals of signalsource 913 and the phase of the output signals of signal source 914 arecontrolled in order to keep the open-loop phase shift at apre-determined value. Negative feedback loop 810 settles most rapidlywhen the open-loop phase shift is 180°. In other words, the time up towhen measurements start is shortened. Consequently, the phase of theoutput signals of signal source 913 and the phase of the output signalsof signal source 914 are controlled so that the value obtained bysubtracting 180° from the open-loop phase shift is desired open-loopphase shift φ. By compensating for open-loop loss and controlling theopen-loop phase shift as described above, the settling time of negativefeedback loop 810 is uniform and can be universally shortened at all ofthe measurement signal frequencies, regardless of the impedance ofdevice under test 100 that is connected, the ground capacitance of waferinterface device 700, the total ground capacitance of cable 510, and thefrequency of the measurement signals, and therefore the output signal ofcurrent-to-voltage converting apparatus 800 settles rapidly. As abovementioned, control of the open-loop phase shift is performed bycontrolling the phase of the-output signals of signal source 913 and thephase of the output signals of signal source 914. Control of theopen-loop phase shift can also be performed by controlling the phase ofthe output signals of signal source 973 and the phase of the outputsignals of signal source 974.

Next, at step 50, negative feedback loop 810 is closed and the impedanceof device under test 100 is measured. Specifically, switches 951 and 952are brought to the T side and variable gain amplifier 941 and mixer 971are connected and variable gain amplifier 942 and mixer 972 areconnected, respectively. Cosine signals are output from signal source974. Switch 980 and switch 990 are both turned off. The output signalE_(dut) of buffer 550 and the output signal E_(rr) of buffer 560 aremeasured by vector voltmeter 400. Furthermore, the vector ratio of themeasured signal E_(dut) and the signal E_(rr) is calculated and theimpedance of device under test 100 is calculated from the calculatedvector ratio and the resistance of range resistor 330.

Next, at step 60, the calculated impedance is output to the displayscreen (not illustrated), or is output to the printer (not illustrated)that is connected to impedance measuring apparatus 20 or the like.

The above-mentioned embodiment of the present invention is only oneembodiment that explains the present invention according to the Scope ofthe Patent Claim, and it is clear to experts in the field that a varietyof modifications are possible within the claimed scope of the Scope ofthe Patent Claim. Finally, several embodiments of the present inventionare given below, underscoring the possibility of broad application ofthe present invention.

A current-to-voltage converting apparatus characterized in that it is acurrent-to-voltage converting apparatus connected to an element or acircuit having a first terminal connected to a signal source, with thiscurrent-to-voltage converting apparatus comprising: a feedbackamplifier, which is connected to a second terminal of this element orthis circuit and keeps this second terminal at virtual ground, and whichconverts the current signals that flow to this element or this circuitto voltage signals and outputs these signals, means for opening thefeedback loop of this feedback amplifier and measuring the open-looploss of this feedback loop, and a compensating amplifier, whichcompensates for this open-loop loss.

The current-to-voltage converting apparatus as discussed above,characterized in that it further comprises: means for measuring theopen-loop phase shift of this feedback loop when this feedback loop isopen; and control means for keeping this open-loop phase shift at apre-determined value.

When this feedback loop is open or the open-loop loss of this feedbackloop is measured, the output of this signal source is controlled so thatit becomes zero or a direct-current signal.

The feedback amplifier preferably comprises a modulation-typenarrow-band amplifier, and this narrow-band amplifier comprises a phasesensitive detector, filters, and a vector modulator.

The compensating amplifier is placed in between the phase sensitivedetector and the vector modulator.

The control means controls the phase difference between the signal thatis applied to the phase sensitive detector and the signal that isapplied to the vector modulator.

The feedback loop is opened by being opened in between the phasesensitive detector and the vector modulator.

The feedback amplifier further comprises a null detector and a feedbackcircuit, the null detector is connected to the second terminal and thesignals that are input to the null detector are converted to voltagesignals by the null detector, the narrow-band amplifier resolves thisconverted voltage signal into an in-phase component and anquadrature-phase component using the phase sensitive detector, filtersthis in-phase component and this quadrature-phase component using theserespective filters, vector modulates this filtered in-phase componentand this filtered quadrature-phase component using this vectormodulator, and outputs the vector voltage signals and the feedbackcircuit inputs these vector signals to the null detector.

The element or circuit is a capacitive element or capacitive circuit.

An impedance measuring apparatus which comprises: a signal sourceconnected to a first terminal of a device under test, a feedbackamplifier, which is connected to a second terminal of the device undertest and keeps the second terminal at virtual ground, and which convertscurrent signals that flow to this device under test to voltage signalsand outputs these signals, means for opening the feedback loop of thisfeedback amplifier and measuring the open-loop loss of this feedbackloop, a compensating amplifier, which compensates this open-loop loss,and means for measuring the vector voltage ratio between the voltagesignals between the first terminal and the second terminal and theoutput signals of the feedback amplifier, wherein it measures theimpedance of the device under test from this vector voltage ratio.

The impedance measuring apparatus further comprising: means formeasuring the open-loop phase shift of the feedback loop when thisfeedback loop is open; and control means for keeping the open-loop phaseshift at a pre-determined value.

The feedback loop is open or the open-loop loss of this feedback loop ismeasured, the output of the signal source is controlled so that itbecomes zero or a direct-current signal.

The feedback amplifier comprises a modulation-type narrow-bandamplifier, and this narrow-band amplifier comprises a phase sensitivedetector, filters, and a vector modulator.

The compensating amplifier is placed in between the phase sensitivedetector and the vector modulator.

The control means controls the phase difference between the signal thatis applied to the phase sensitive detector and the signal that isapplied to the vector modulator.

The feedback loop is opened by being opened in between the phasesensitive detector and the vector modulator.

The feedback amplifier further comprises a null detector and a feedbackcircuit, this null detector is connected to the second terminal and thesignals that are input to the null detector are converted to voltagesignals by the null detector, the narrow-band amplifier resolves thisconverted voltage signal into an in-phase component and anquadrature-phase component using the phase sensitive-detector, filtersthis in-phase component and this quadrature-phase component using theserespective filters, vector modulates this filtered in-phase componentand this filtered quadrature-phase component using the vector modulator,and outputs the vector voltage signals, and the feedback circuit inputsthese vector signals to the null detector.

The element or the circuit is a capacitive element or capacitivecircuit.

As previously described in detail, a current-to-voltage convertingapparatus connected to an element or a circuit having a first terminalconnected to a signal source comprises a feedback amplifier, which isconnected to a second terminal of this element or this circuit and keepsthe second terminal at virtual ground, and which converts the currentsignals that flow to this element or this circuit to voltage signals andoutputs these signals; means for opening the feedback loop of thisfeedback amplifier and measuring the open-loop loss of this feedbackloop; and a compensating amplifier, which compensates for this open-looploss, and therefore, the settling time of this feedback loop isshortened.

In addition, it comprises means for measuring the open-loop phase shiftof the feedback loop when this feedback loop is open and control meansfor keeping this open-loop phase shift at a pre-determined value, andtherefore, the settling time of this feedback loop is further shortened.

The result of shortening the settling time of this feedback loop issimilarly obtained with the impedance measuring apparatus comprising theabove-mentioned current-to-voltage converting apparatus. That is, theimpedance measuring apparatus comprises a signal source connected to afirst terminal of a device under test; a feedback amplifier, which isconnected to a second terminal of this device under test and keeps thesecond terminal at virtual ground, and which converts current signalsthat flow to this device under test to voltage signals and outputs thesesignals; means for opening the feedback loop of this feedback amplifierand measuring the open-loop loss of this feedback loop; a compensatingamplifier, which compensates this open-loop loss; and means formeasuring the vector voltage ratio between the voltage signals betweenthe first terminal and the second terminal and the output signals ofthis feedback amplifier. Therefore, the settling time of this feedbackloop can be shortened and high-speed measurement is possible.

Moreover, the vector measuring apparatus comprises means for measuringthe open-loop phase shift of this feedback loop when this feedback loopis open and control means for keeping this open-loop phase shift at apre-determined value. Therefore, the settling time of this feedback loopis further shortened and measurements can be conducted more rapidly.

For instance, when the device under test is a capacitor of 10 pF and theground capacitance of the wafer interface device is 1,000 pF or higherand the impedance of the device under test is measured at a measurementsignal of 100 kHz, the measurement time of the impedance measuringapparatus of the present invention proceeds at least three times morerapidly than that of a conventional apparatus.

1-9. (canceled)
 10. An impedance measuring apparatus which comprises: asignal source connected to a first terminal of a device under test, afeedback amplifier, which is connected to a second terminal of saiddevice under test and keeps said second terminal at virtual ground, andwhich converts current signals that flow to said device under test tovoltage signals and outputs these signals, means for opening thefeedback loop of said feedback amplifier and measuring the open-looploss of said feedback loop, a compensating amplifier, which compensatessaid open-loop loss, and means for measuring the vector voltage ratiobetween the voltage signals between said first terminal and said secondterminal and the output signals of said feedback amplifier, whereby itmeasures the impedance of said device under test from said vectorvoltage ratio.
 11. The impedance measuring apparatus according to claim10, further comprising: means for measuring the open-loop phase shift ofsaid feedback loop when said feedback loop is open; and control meansfor keeping said open-loop phase shift at a pre-determined value. 12.The impedance measuring apparatus according to claim 10, wherein saidfeedback loop is open or the open-loop loss of said feedback loop ismeasured, the output of said signal source is controlled so that itbecomes zero or a direct-current signal.
 13. The impedance measuringapparatus according to claim 10, wherein said feedback amplifiercomprises a modulation-type narrow-band amplifier, and said narrow-bandamplifier comprises a quadrature detector, filters, and a vectormodulator.
 14. The impedance measuring apparatus according to claim 13,wherein said compensating amplifier is in between said quadraturedetector and said vector modulator.
 15. The impedance measuringapparatus according to claim 13, wherein said control means controls thephase difference between the signal that is applied to said quadraturedetector and the signal that is applied to said vector modulator
 16. Theimpedance measuring apparatus according to claim 13, wherein saidfeedback loop is opened by being opened in between said quadraturedetector and said vector modulator.
 17. The impedance measuringapparatus according to claim 13, wherein said feedback amplifier alsocomprises a null detector and feedback circuit, said null detector isconnected to said second terminal and the signals that are input to thenull detector are converted to voltage signals by the null detector,said narrow-band amplifier resolves said converted voltage signal intoan in-phase component and an quadrature-phase component using saidquadrature detector, filters said in-phase component and saidquadrature-phase component using said respective filters, vectormodulates said filtered in-phase component and said filteredquadrature-phase component using said vector modulator, and outputs thevector voltage signals, and said feedback circuit inputs said vectorsignals to said null detector.
 18. The impedance measuring apparatus inclaim 10, wherein said element or said circuit is a capacitive elementor capacitive circuit.